Project Summary Low high-density lipoprotein cholesterol (HDL-C) is a major risk factor for atherosclerotic cardiovascular disease (ASCVD), the leading cause of death. However, strategies to improve atheroprotection by raising HDL-C levels have failed to improve outcomes. Macrophage- specific cholesterol efflux is the first critical step of reverse cholesterol transport, a key atheroprotective pathway. Cholesterol efflux is inversely associated with incident ASCVD events; however, the mechanisms that underlie variation in cholesterol efflux are unknown. There is a critical need to identify factors that regulate cholesterol efflux to advance understanding of the reverse cholesterol transport pathway, a biological process that has broad implications across a variety of disease states including atherosclerosis. The overall objective of this proposal is to systematically ascertain the genetic and molecular factors governing variation in cholesterol efflux. We propose to carry out our objective by using two large, population- based multi-ethnic cohorts, the Dallas Heart Study (DHS) and the Multi-Ethnic Study of Atherosclerosis (MESA) to pursue the following aims: 1) determine the contribution of genetic factors to inter-individual variability in cholesterol efflux and whether these factors are causally associated with ASCVD; and 2) identify the metabolite and protein signature of cholesterol efflux in a sex- and ethnicity-specific manner. We expect to characterize the genetic, metabolic, and protein regulators of cholesterol efflux to support future studies targeting manipulation of the reverse cholesterol transport pathway.